1. Field of the Invention
This invention relates to a method and specimen configuration for measuring the true (as opposed to "apparent") fracture toughness on small sized core specimens. More specifically, the invention relates to a modified ring test that accurately measures rock toughness using small size specimens.
2. Description of the Prior Art
It is generally accepted that rock fracturing is of fundamental importance in several oil and gas well completion services: drilling, hydraulic fracturing (i.e., the pumping of fluids into a well in order to fracture the reservoir and increase hydrocarbon production) and borehole stabilizing (i.e., preventing the collapse of the well bore during completion, stimulation and production). As such, an accurate and reliable quantitative measurement of rock fracture toughness is necessary to perform rock fracture simulation studies utilizing contemporary mathematical modeling such as pseudo-three dimensional simulators and the like.
The stress intensity factor approach is commonly used in three-dimensional hydraulic fracturing models. The stress intensity factor is a measurement of the stress singularity near the crack tip. The pertinent criterion for propagation is that fracture will propagate once the stress intensity factor, K.sub.I, is equal to or greater than a critical value, K.sub.Ic. K.sub.Ic is referred to as the fracture toughness and is a material property. Generally, the stress intensity factor is viewed as a function of the geometry of the crack, the geometry of the body, and the loading parameters (i.e., fluid pressure in the fracture and confining pressure) while the toughness is viewed as a function of the rock type only. For a more detailed explanation of the stress intensity factor, see G. R. Irwin, "Analysis of Stresses and Strains Near the End of a Crack Traversing a Plate", Journal of Applied Mechanics, 24 (1957): 361-4 and G. R. Irwin & R. deWit, "A Summary of Fracture Mechanics Concepts", Journal of Testing and Evaluation, 11 (1983): 56-65 herein incorporated by reference for such purpose.
The stress intensity approach assumes that the rock behaves as a linear elastic material. This assumption is acceptable if the zone of non-linear behavior, ahead of the fracture tip, is small in comparison to the other geometric dimensions, including the crack length. The non-linear zone is due to microcrack formation as a consequence of extreme stress concentrations in the immediate vicinity of the crack tip. Experimental results on rocks have shown that linear elasticity is an acceptable assumption if the crack length is greater than 100 to 200 millimeters. These dimensional limitations imply that field scale fractures (of great length) can be evaluated using linear elastic theory; whereas, in laboratory testing, the determination of K.sub.Ic is carried out on more modest length specimens; hence care must be taken to avoid or account for non-linearity. In other words, in order to obtain a valid measurement of toughness in the laboratory, the size of the process zone (i.e., the non-linear zone) should be small compared to the initial notch length. Such a requirement is often difficult to meet, particularly when a typical rock core is used as the specimen and consequently, accurate toughness determinations may not be achieved. For example, when using the prior art three point bending tests and/or center notched panel tests (see FIG. 4), a relatively large process zone ahead of the crack tip makes the apparent toughness size dependent. Consequently, underestimation of the actual toughness, K.sub.Ic, when using the prior art methods on subsidized specimens, will frequently occur.
Thus, prior to the present invention, a need for a test procedure that measures the fracture toughness in a manner that is virtually independent of the test configuration and, in particular, the size of the specimen, existed. Ideally, such a test procedure would further allow for determination of K.sub.Ic at representative in-situ stresses and environmental conditions. The modified ring test method and novel specimen configuration, according to the present invention, is felt to satisfy these needs.